linux-kernel - Re: [PATCH] vsprintf: Fix memory barriers of ptr_key to have_filed_random_ptr

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Date:   Tue, 15 May 2018 18:41:17 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        tcharding <me@...in.cc>
Subject: Re: [PATCH] vsprintf: Fix memory barriers of ptr_key to
 have_filed_random_ptr_key

On Tue, 15 May 2018 15:31:37 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > we could always do this:  
> 
> Ugh. I think I prefer the barriers.

But the ugliness is only at boot up, where the early printing is
important. The benefit of this is here:

@@ -1709,7 +1717,7 @@ static char *ptr_to_id(char *buf, char *end, void *ptr, struct printf_spec spec)
        unsigned long hashval;
        const int default_width = 2 * sizeof(ptr);
 
-       if (unlikely(!have_filled_random_ptr_key)) {
+       if (static_branch_unlikely(&not_filled_random_ptr_key)) {
                spec.field_width = default_width;
                /* string length must be less than default_width */
                return string(buf, end, "(ptrval)", spec);

Because not only do we not add any barriers, this code basically
becomes a nop once the ptr_key is filled, and this should have a small
speed up in execution (although I have not done any benchmarks).

-- Steve